Despite obvious technologic advances in imaging, the chest radiograph remains the most frequent initial method used in assessing suspected mediastinal pathology.

Whenever possible, the initial evaluation of a patient with suspected mediastinal pathology should include posteroanterior (PA) and lateral radiographs. Portable radiography has a variable magnification, which reduces the reliability of interpretation of mediastinal widening. Also, the classification of mediastinal masses is based on anterior to posterior position of the mass; thus, a lateral chest radiograph is important in limiting the differential diagnosis. PA and lateral radiographs are performed at a fixed target-to-film distance, 72 inches, and often photo-timed. This usually results in optimal and reproducible imaging characteristics. Traditionally, images have been acquired by standard photographic film-screen systems. More recently a variety of alternative imaging methods have been developed, including digital radiography, beam equalization radiography and others. Korner¹⁹ has published an excellent review of this topic. In general, these newer techniques provide similar information to film-screen radiographs and in some instances produce improved visualization of mediastinal structures.

The computed tomography (CT) examination is in general the most accurate and reliable noninvasive method of mediastinal evaluation.

Since its introduction in 1988, spiral (helical) CT technology has revolutionized CT imaging. With earlier scanners, an image was obtained, followed by incremental movement of the CT table, followed by a second image and second table movement until the area of interest was completely imaged. With spiral CT, images are generated continuously as the CT table, and therefore the patient, slowly moves through the scanning plane. Thus, instead of images being obtained as a series of stacked rings, they are obtained as a continuous spiral. This has the
disadvantage of complicating the mathematics of image reconstruction, but it has the major advantage of dramatically increasing the speed of imaging. Prior scanners typically had a 1-second scan time per image and a 1- to 2-second delay during table and x-ray tube repositioning. A single spiral typically takes 1 second, with no delay between images. Spiral CT technology has been significantly advanced with the introduction of scanners using multiple detectors. In 1992, the first multidetector CT scanners used two detectors, followed in 1998 by scanners with four detectors. Current-generation scanners employ 16 or more detectors, with 64-detector scanners being considered state of the art for the assessment of airway and vascular disease. Significantly reduced scanning times have further improved spatial resolution with reduced cardiac and respiratory motion artifacts. Imaging during the optimal contrast bolus peak is facilitated, thus improving visualization of the aorta and pulmonary arteries. These factors permit high-quality multiplanar reconstructions that assist in CT arteriographic studies and three-dimensional volume rendering. Flohr¹³ has reviewed the applications of multidetector CT.

Magnetic resonance imaging (MRI) of the thorax is limited by poor imaging of the lung parenchyma. In addition, MRI is sensitive to motion-related artifacts, which are often present because of normal cardiac and respiratory motion. As a result, MRI is usually restricted to situations where other imaging, particularly CT, does not adequately evaluate a given problem. As a consequence, there is no standard imaging protocol for MRI. Each examination should be specifically tailored to the clinical indication.

Despite these limitations, MRI, when performed correctly, provides excellent evaluation of mediastinal structures. Advantages include nonaxial, multiplanar capabilities. Disorders that primarily manifest in a sagittal or coronal direction—for example, apical chest masses or tracheal diseases—may be better evaluated by MRI than CT. In addition, MRI provides better soft tissue characterization than CT and, in select instances, may be more accurate in delineating the composition of a mass. Last, MRI has intrinsic sensitivity for vascular abnormalities without the necessity of intravenous contrast. Consequently it is often the study of choice for imaging of the aorta or central pulmonary arteries.

Chapter 11 provides details of specific clinical indications in which MRI is currently most useful and provides information regarding imaging protocols.

Many thyroid masses are incidentally detected on chest radiographs, CT, and MRI because they are common asymptomatic masses. Most thyroid masses are readily palpated, and in some instances it is most expedient to diagnose them directly with needle aspiration rather than with imaging. Some thyroid masses are occult on chest radiographs; however, many thyroid masses are evident as anterior mediastinal masses. The most sensitive and specific plain film manifestation of a thyroid mass is focal deviation of the trachea at the level of the clavicles. It is rare for nonthyroid tumors to deviate the trachea, although this is often seen with the rare large parathyroid cysts, as reported by Shields and Immerman.³⁸ Larger thyroid masses also produce widening of the superior mediastinum. In most cases the particular type of thyroid mass (e.g., adenoma, carcinoma, lymphoma, multinodular goiter) is not diagnosable on the basis of the chest radiograph alone. If the mass is large and has remained stable over years, it is most likely a multinodular goiter. When a specific diagnosis is necessary, needle aspiration is the test of choice.

When necessary, the imaging study of choice is often a radionuclide thyroid scan, addressed in Chapter 167. Ultrasonography may be useful in demonstrating that a thyroid mass is a simple cyst and in guiding needle aspiration in patients with nonpalpable nodules. Simple cysts are seen in ultrasound imaging as sharply marginated, thin-walled, round or oval structures with no internal echoes and with through transmission of sound. In general, CT and MRI are not indicated for the evaluation of thyroid masses. However, they are useful in distinguishing between thyroid masses and other mediastinal masses. Glazer and associates¹⁵ were among the first to show that CT can readily demonstrate the contiguity of a mass with the thyroid gland and thereby indicate that the mass is of thyroid origin. These investigators, as well as von Schulthess and colleagues,⁴⁵ demonstrated distortion of the great vessels anteriorly and laterally, thereby correctly confirming the placement of these masses.

The most common thyroid mass is the multinodular goiter. This benign tumor of the thyroid gland is commonly seen in elderly patients. It has similar features in all cross-sectional imaging. It appears as a multilobular mass with multiple rounded nodular areas, and there are usually associated cysts and foci of calcification. These nodules and cysts may be of widely varying sizes.

Thyroid adenomas appear as discrete nodular masses within the substance of the thyroid. It is not possible to distinguish benign from malignant masses on the basis of imaging studies; histologic or cytologic analysis of a solitary thyroid mass is necessary to determine whether it is malignant. A smooth, well-defined contour favors a benign adenoma, whereas an ill-defined infiltrating mass almost always represents a thyroid carcinoma.

Technetium 99m (^99mTc)–sestamibi scintigraphy is currently the most sensitive imaging procedure for parathyroid adenomas, which can be present anywhere from the base of the tongue to the surface of the pericardium; intrathymic location is common. (See Chapter 167.) MRI may also be useful for identifying mediastinal parathyroid adenomas in patients with unexplained hypercalcemia. These usually appear as small (<3 cm), well-defined masses in the anterior mediastinum; they are isointense to muscle on T1-weighted images and very hyperintense to muscle on T2-weighted images. Lee and coworkers²² have shown that in cases of occult parathyroid adenomas, the combination of both MRI and ^99mTc-sestamibi scintigraphy is often more sensitive for the detection of adenomas than either imaging study alone.

Lymphangiomas or cystic hygromas are rare benign tumors of the lymphatic system. They characteristically arise in the neck but can rarely be completely intrathoracic or have an intrathoracic component. Chest radiographs demonstrate a nonspecific anterior or middle mediastinal mass. On cross-sectional imaging, they often appear as unilocular or multilocular cystic masses with a variable solid component and occasional areas of hemorrhage, as noted by Miyake and coworkers.²⁷ Although histologically benign, these tumors often infiltrate around the mediastinal organs, thus preventing complete surgical excision.

The most common tumors of the anterior mediastinum and thymus are Hodgkin’s and non-Hodgkin’s lymphomas. They may appear as isolated anterior mediastinal masses, and they may be associated with pulmonary parenchymal infiltrates or with adenopathy in the mediastinum, neck, axilla, or abdomen (Fig. 166-5). Early lymphomatous involvement of the thymus may be occult on chest radiographs, but on cross-sectional imaging it appears as a triangular mass, mimicking the shape of the normal thymus. As the mass becomes larger, it becomes more rounded or amorphous. Cavitation or cystic degeneration is rare. The tumor may grow directly into the lung or surrounding structures of the chest wall. This feature is characteristic of lymphomas, and its presence makes a lymphoma more likely than another anterior mediastinal mass.

Teratomas and other mediastinal germ cell tumors may be impossible to distinguish from thymomas by imaging criteria alone. Both can appear as discrete or infiltrating anterior mediastinal masses. On cross-sectional imaging, both may appear as solid masses and may contain calcifications. Teratomas, however, more frequently than thymomas, show areas of low density within the mass because of either cystic or fatty components. In a study by Moeller and colleagues,²⁸ 63% of mature mediastinal teratomas contained solid, fluid, and fatty elements with or without associated areas of calcification. This combination of findings is virtually pathognomonic of a benign mature teratoma, the most common mediastinal germ cell tumor, accounting for 75% of all mediastinal germ cell tumors. Malignant germ cell tumors—such as seminoma, choriocarcinoma, and yolk sac tumors—often have a less specific, homogeneous soft tissue attenuation. Germ cell tumors more commonly occur in younger patients, an average of 24 years of age in the study by Moeller and colleagues,²⁷ whereas thymomas are most often seen in middle-aged or elderly patients.

The chest radiograph is insensitive to the detection of mediastinal and hilar lymphadenopathy. Lymph nodes as large as 2 or 3 cm may be occult on chest radiographs. When present, lymphadenopathy may appear as nonspecific widening of the mediastinum, thickening of the right paratracheal stripe, filling in of the normal AP window concavity, or as subtle opacity in the subcarinal region. Hilar adenopathy appears as an enlarged hilum on chest radiographs. The hilum often has a lobulated contour when adenopathy is present (Fig. 166-6).

Perhaps the greatest utility of CT in evaluating the mediastinum has been in the detection of lymphadenopathy. The extent and distribution of lymphadenopathy is accurately assessed with CT. Lymph nodes appear as discrete or confluent regions of soft tissue attenuation separable from other mediastinal structures, particularly when intravenous contrast has been administered (Fig. 166-6). Lymphadenopathy may contain regions of lower attenuation because of necrosis or high attenuation due to calcification from granulomatous disease, chemotherapy, or radiation therapy. The identification of lymph nodes does not indicate their histologic characteristics or even the presence of disease. Anatomic and pathologic studies reported by Genereux and Howie¹⁴ of patients with no known chest disease frequently identified lymphadenopathy in all regions of the mediastinum. Lymph node size tended to vary with location; however, 95% of normal lymph nodes were less than 11 mm in diameter. Enlarged lymph nodes may have a variety of causes, including metastatic tumor, lymphoma, sarcoidosis, and other granulomatous or inflammatory causes.

Metastatic lung cancer is the most common cause of malignant adenopathy. A short-axis dimension of 10 mm is used as the criterion for reporting potentially abnormal lymph nodes in lung cancer staging studies. It has become apparent that assessment of nodal size by CT is not sufficiently accurate for routine clinical staging, because significant numbers of small lymph nodes contain microscopic metastasis and some large lymph nodes are free of metastatic disease. Series by Whittlesey⁴⁸ and McKenna and associates²⁵ demonstrated a 25% to 30% chance for recovering metastases in lymph nodes measuring 11 to 19 mm and a similar chance for not revealing metastatic disease in nodes measuring >20 mm. Nonneoplastic nodal enlargement may be a manifestation of inflammatory changes related to postobstructive pneumonia. For this reason, squamous cell carcinoma has the highest likelihood of being associated with benign nodal enlargement. Shimoyama and coworkers³⁹ have suggested that the morphology of hilar lymph nodes may help in separating benign from malignant lymph nodes. In their preliminary study of lung cancer patients, hilar lymph nodes with a convex margin to the lung parenchyma had an 87% sensitivity and 88% specificity for malignant adenopathy, regardless of node size. The reliability of this finding must be validated in future larger studies. Thus,
at present CT is best used as a tool to guide surgical staging to the most likely sites of metastatic spread. In addition, some clinicians believe that the absence of radiographically detectable lymph nodes on a good-quality CT scan obviates the need for staging mediastinoscopy prior to definitive surgical resection.

Positron emission tomographic (PET) scanning, with its ability to distinguish between metabolically active and quiescent tissues, has proved increasingly beneficial in the noninvasive staging of the mediastinum. (See Chapter 167.)

Metastatic adenopathy from tumors other than lung cancer is uncommon. Renal, testicular, and breast carcinomas, as well as melanoma, are some of the extrathoracic malignancies more likely to develop malignant mediastinal adenopathy.

Lymphoma is a common cause of mediastinal adenopathy, most frequently producing abnormalities in the anterior and middle mediastinal compartments. Lymphomas also commonly involve hilar, axillary and supraclavicular lymph nodes. When large, these may be seen on chest radiographs. However, chest radiographs are insensitive to smaller but still enlarged mediastinal nodes, which may be demonstrated by CT and MRI. Castellino and coworkers⁸ noted that thoracic CT detected disease that had been occult on chest radiographs in 9% of patients with newly diagnosed non-Hodgkin’s lymphoma and detected increased extent of disease relative to chest radiographs in an additional 27%. Salonen and coworkers³⁵ have reported that CT-detected mediastinal involvement of lymphoma is missed by chest radiography in 39% of cases. As a consequence, cross-sectional imaging is commonly used to monitor mediastinal disease progression during chemotherapy. In follow-up examinations, persistent, stable soft tissue in the anterior mediastinum indicates residual fibrosis without persistent tumor and does not require further therapy, a concept emphasized by Thomas and colleagues⁴² in their report of surveillance CT in the treatment of Hodgkin’s lymphoma.

Mildly enlarged mediastinal nodes (short-axis diameter of 10 to 20 mm) may be seen in a wide variety of inflammatory conditions of the thorax, including pneumonias, interstitial lung disease, and many other conditions. Therefore, in the setting of a known inflammatory process in the thorax, the clinician should remain
unconcerned with the presence of a few mildly enlarged lymph nodes. For example, nodal enlargement is a consistent finding in other chronic infiltrative interstitial diseases, including scleroderma and usual interstitial pneumonitis. However, the presence of many mildly or of markedly enlarged lymph nodes (>20 mm) should prompt an investigation for their cause.

Sarcoidosis is among the most common nonneoplastic causes of mediastinal and hilar lymphadenopathy. Sarcoidosis characteristically causes moderate to marked enlargement of bilateral hilar and right paratracheal lymph nodes (see Fig. 166-6). These are usually readily apparent on chest radiographs. Many patients have concurrent interstitial lung disease. (See Chapter 10 for more details regarding thoracic involvement with sarcoidosis.)

Primary tuberculosis (TB) most commonly presents with a focal alveolar pneumonia and associated unilateral hilar or mediastinal adenopathy. Because most primary tuberculosis is unrecognized clinically, it is relatively uncommon to image this condition in the United States. Adenopathy is rare in postprimary or reactivation TB and in nontuberculous infections in immunocompetent patients. However, in patients infected with human immunodeficiency virus (HIV), both tuberculous and nontuberculous mycobacterial infections commonly result in mediastinal and hilar adenopathy (see Fig. 166-7). Prominent necrosis manifests as zones of low density within involved nodes.